In recent years carbonless copying systems have come into wide-spread usage for business records, printout paper for computers and the like. There is considerable patent literature pertaining to the preparation of pressure-sensitive record material utilizing, in various physical associations or arrangements, a color-forming dye precursor compound, preferably colorless, in conjunction with an image-developing sheet carrying a color-developer material capable of reacting with the dye precursor to produce a visibly colored reaction product. The dye precursor is generally selected from one of a number of chemical classes and contained in some kind of pressure-rupturable or releasable state. The color-developer generally used includes clays such as attapulgite, acid clay, active clay, zeolite and bentonite.
Colorless copying systems operating on the basis of the donor-acceptor mechanism have been plagued with loss of coloration or change in coloration of the developed dye image with exposure to light and ordinary atmosphere. One solution to the instability problem involved the judicious combination of plural dye precursor compounds. One dye was a relatively fast reacting, but relatively fugitive dye, while a second dye was converting from colorless to colored state over prolonged periods of time and, optimally, under essentially the same conditions that accelerated the fading of the first dye. Exemplary of such systems are U.S. Pat. No. 2,505,470 and U.S. Pat. No. 3,455,721. This approach cannot be considered an ideal answer since it necessitates the careful selection of precursors to achieve a compatible combination with appropriately balanced fading characteristics so that the image does not at any time pass through an essentially colorless stage. In addition, the color of the image tended to change with time, which was often disconcerting to the user, and the range of colors that could be conveniently made available is quite limited.
British Pat. No. 1,356,402 published June 12, 1974 discloses a color-developer system which provides colored images with increased resistance to discoloration or fading which system comprises at least one monomeric phenolic compound together with one or more clays. Both the monomeric phenolic compound and the clay are said to act as color developers. The monomeric phenolic compounds disclosed are substituted phenols and polyhydric phenols such as p-cresol, p-phenylphenol, p-bromophenyl-phenols, biphenols, such as o,o'-biphenol, 2,4-xylenol, 2,4-diphenyl phenol, 2-chloro-4-phenylphenol, 2,3,5-trimethylphenol, tetramethyl phenols, 4-phenylpyrocatechol, 4,4'-bi-o-cresol, x,x'diphenyl-4,4'-bi-o-cresol, biphenyl tetrols, 2,4'-methylene diphenol, methylene diresorcinols, 2,2'-methylene-di-p-cresol, methylene bis-(benzyl phenols), p,p'-isopropylidenediphenol, methylene bis-(phenyl phenols), methylene bis-(halophenols), and 4,4'-thiodiphenol.